The use of fabrics woven from polymeric fibers and fiberglass to reinforce structural members is well known and the use of these materials has proven satisfactory over a number of years. However, in a variety of specialty applications which include protective enclosures for electronic equipment that receives and transmits electromagnetic signals, it is quite important that the protective enclosure interferes with the transmission to only the minimal extent possible. Thus, it is a primary object of the present invention to provide a reinforcing fabric with a low dielectric constant that will be particularly suitable for reinforcing such structures.
Another important factor in developing reinforcing fabrics is to weave the fabric in a manner so that the fabric can be stretched to conform to the desired shape of the article to be formed without wrinkles, bulges, or distortions. Such fabrics are sold under the brand name, "Conform Fabrics.RTM.". Conform Fabrics and fabrics of other weave patterns are disclosed in U.S. Pat. No. 5,102,725 which issued on Apr. 7, 1992 to Martha Knox et al and is incorporated herein by reference.
Certain polymeric filaments that can be spun and woven into fabrics have low dielectric constants but tend to soften and weaken when a heated molten thermosetting polymeric material is applied to the fabric as a coating; or, if the coating is applied by immersing the fabric in a bath and then cured by heating at an elevated temperature for an extended period, the curing temperature may be in the range of the glass transition temperature of the polymeric filaments. Thus, it is another object of the present invention to provide a low dielectric fabric with improved strength to withstand the application of heated coatings and curing temperature.
In the past, reinforcing of an aramid fiber by twisting with fiberglass has been proposed as disclosed in U.S. Pat. No. 4,528,223 which issued on Jul. 9, 1985 to Kumazawa et al. However, aramid polymers tend to have a relatively high dielectric constant and tend to absorb moisture which further raises the dielectric constant. Thus, it is yet another object of the present invention to provide a fabric with added strength, lower dielectric constant, and improved bonding to applied coatings than that which can be provided by such prior art fibers and fabrics.
Also, available and useful polyaramid fibers such as those sold under the Kevlar.RTM. brand and polyethylene fibers sold under the Spectra.RTM. brand must be plasma etched in order to increase the bondability of their surfaces to a satisfactory level. Furthermore, the plasma etching effect has a very limited shelf life, i.e., the surface impregnation and bonding improvement are time dependent so that the fiber and/or fabric so treated must be woven and coated without prolonged delay or storage. Accordingly, it is another object of the present invention to avoid the necessity of plasma etching.
These and other objects of the present invention will become generally apparent to those skilled in the art from the following Summary of the Invention and Detailed Description.